This invention relates in general to the field of electronic signal processing and more particularly to a system and method for recovering a pilot tone in a local multipoint distribution system signal.
A Local Multipoint Distribution System (LMDS) is a point to multipoint radio frequency communication system. It operates in the United States using frequency bands between 26 GHz and 29 GHz. The specific frequencies assigned to LMDS varies by country and is generally in the 10 GHz to 40 GHz frequency range. The frequency range set aside for LMDS requires a line of sight between the transmitting device and receiving device. LMDS may be used to provide and distribute telecommunication services, data communication services, and video services.
The LMDS head end and customer premises operate on an intermediate frequency. The intermediate frequency allows existing modems to communicate with digital electronic devices. Prior to transmission at the head end, the intermediate frequency signal is translated, or upconverted, to the appropriate LMDS frequency band. A receiver at the customer premises receives the LMDS signal and translates, or downconverts, the LMDS signal from the LMDS frequency band to an intermediate frequency suitable for processing at the customer premises. Communications in the gigahertz frequency range requires signals with very stable frequencies. The head end, or node, has very expensive oscillators for maintaining stable frequencies. The customer premises equipment needs lower cost equipment which generally cannot maintain a stable frequency.
A constant signal at a specified frequency is used to synchronize the oscillators in the customer premises equipment with the head end transmission equipment. This constant signal at a specified frequency is referred to as the pilot tone. The pilot tone is generally a low amplitude, fixed frequency signal at the lower end of the frequency range assigned for LMDS communications. The customer premises equipment locates the pilot tone in the signal received from the LMDS head end and synchronizes its equipment to generate a signal at the same frequency. After synchronization of the oscillators in the customer premises equipment, the customer premises equipment can extract the LMDS signals in the remaining LMDS bandwidth.
Conventional pilot tone recovery systems utilize surface acoustic wave (SAW) filters to locate the pilot tone signal within the LMDS transmission. SAW filters can be expensive and difficult to fabricate. In addition, SAW filters cannot be formed on a monolithic integrated circuit. Therefore, it is desirable to effectively and efficiently recover the pilot tone within a LMDS transmission.
From the foregoing, it may be appreciated that a need has arisen for an effective and efficient pilot tone recovery system. In accordance with the present invention, an improved system and method for recovering a pilot tone in an LMDS signal are provided which substantially eliminate or reduce disadvantages and problems associated with conventional pilot tone recovery systems.
According to an embodiment of the present invention, there is provided a method for recovering a pilot tone in an LMDS signal that includes receiving an intermediate frequency signal that contains a pilot tone frequency and removing frequencies surrounding the pilot tone frequency. A pilot tone signal is then generated from the pilot tone frequency. The frequency of the pilot tone signal is then divided by a specified number and compared to a stable frequency signal to determine a frequency difference between the divided pilot tone signal and the stable frequency signal. An output frequency of a voltage controlled oscillator is adjusted based on the frequency difference. A system frequency signal is then downconverted to the intermediate frequency signal.
The present invention provides various technical advantages over conventional pilot tone recovery systems. For example, one technical advantage is providing an improved system and method for recovering a pilot tone in an LMDS signal. In particular, the pilot tone recovery system utilizes components capable of fabrication on a monolithic integrated circuit. In addition, the pilot tone recovery system improves the signal to noise ratio of the received pilot tone. Other technical advantages may be readily apparent to one skilled in the art from the following figures, description, and claims.